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A force has both direction & magnitude. Forces are vectors. To completely describe a force you must state how big it is and the direction in which it is acting. We can use simple force detectors to measure and/or detect forces Staple rubber band to index card. Unfold a paperclip. Hang it on the bottom of the rubber band. Attach various weights to the bottom of the paperclip. Note the resting position for each. Simple force detector or accelerometer. Plexiglass casing with colored water sandwiched between. Available commercially.

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Force Detectors The shape of the water indicates the direction of the force. The water has inertia and it remains at rest (or in motion) relative to the container allowing us to see the direction of the force.

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When an object is subject to two or more forces at once, the effect is the cumulative effect of all the forces. Push on a block - does it move? Why/why not? What could we do to make it move? What if we pushed as hard as possible - would that make it move? Identify all the forces involved. When the forces on an object are balanced, the motion of the object does not change. Weight Normal Force Push

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What happens if we remove friction? Providing a cushion of air decreases the force of friction. Allow students to feel frictionless motion! See Altshuler, K. (1989). Human Airpuck, The Physics Teacher,

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When the forces on an object are unbalanced the object will change its motion (that is, it will speed up, slow down, or change direction). Newtons Laws tell us that objects at rest remain at rest, and objects in straight line motion remain in straight line motion, unless acted on by an outside force. How can we use a force detector to see unbalanced forces? Balloon rockets -- tape a balloon to a straw which has a string threaded through it. Release the blown up balloon. What motion is there? What forces are acting on the system? Can you make a 2-stage rocket? A rocket which returns to the launch site?

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When the forces on an object are unbalanced the object will change its motion (that is, it will speed up, slow down, or change direction). Circular motion requires a net force - direction is always changing. What provides the force to keep an object moving in a circle? What happens if that force is removed? The role of gravity in forming and maintaining planets, stars and the solar system. What happens to a marble that is rolling around the edge of a plate when it comes to the end? What are the forces involved here?

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Balloon Powered Circular Motion Attach a balloon to the bottom of a bendy-straw using a rubber band. (blow up balloon once before attaching) Find center of gravity and stick the pin through the straw & into the eraser at that point (be sure the pin is on the outside of the rubber band). Inflate balloon and release air. Bend the straw and repeat.

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Egg into a Sheet Have four students help you hold a bed sheet. They should hold it vertically and taut with one student at each corner. The students on the bottom corners need to create a small gutter to catch the egg. Get a student to throw a raw egg at the sheet as hard as possible. It wont break! Be sure that the sheet is not held directly in front of anything rigid - like a wall!

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Diving Eggs Place a pie tin on 1-3 beakers of water (1/2 filled) so that the edge of the pie tin extends beyond the edge of the table Place toilet paper tubes directly above the beaker(s) Put a raw egg horizontally onto each tube (as shown in next slide) Quickly remove the tray (a broom helps!) The trick:

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Diving Pens Materials: bottle, embroidery hoop, pen with a flat bottom Balance hoop on bottle Balance pen on the hoop (over bottle mouth) Quickly remove the hoop & the pen will go into the bottle The trick to success:

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Other Force Detectors/Accelerometers Cork in a bottle or helium balloon in a car Note the position of the cork & the shape of the water. Net force to the right! No net force, the cork is at rest.

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Other Activities... Egg Cars Students build a car to transport an egg passenger. They build cars with and without safety-belts and air bags. (Eggs sit in paper-cup seats and have rubber band seat-belts and balloon air bags. Students then crash the cars into a wall to see how the safety restraints function. Write-up includes description of the car and safety devices and the physics involved.

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Other Activities... Egg Drop Contests Students are challenged to create a contraption which can safely house an egg for a long distance fall. You can restrict the building materials if desired. Students must develop a sales brochure for their egg carrier which explains how it works as well as sells it to consumers. Students can also build contraptions which will safely catch an egg being dropped from a height.

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California Science Standards Grade 3 Energy and matter have multiple forms and can be changed from one form to another. As a basis for understanding this concept: –Students know energy comes from the Sun to Earth in the form of light. –Students know sources of stored energy take many forms, such as food, fuel, and batteries. –Students know machines and living things convert stored energy to motion and heat. –Students know energy can be carried from one place to another by waves, such as water waves and sound waves, by electric current, and by moving objects. Light has a source and travels in a direction. As a basis for understanding this concept: –Students know sunlight can be blocked to create shadows. –Students know light is reflected from mirrors and other surfaces. –Students know the color of light striking an object affects the way the object is seen. –Students know an object is seen when light traveling from the object enters the eye.

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CA 8 th Grade Physical Science Standards (Forces) Unbalanced forces cause changes in velocity. As a basis for understanding this concept, students know: a. a force has both direction and magnitude. b. when an object is subject to two or more forces at once, the effect is the cumulative effect of all the forces. c. when the forces on an object are balanced, the motion of the object does not change. d. how to identify separately two or more forces acting on a single static object, including gravity, elastic forces due to tension or compression in matter, and friction. e. when the forces on an object are unbalanced the object will change its motion (that is, it will speed up, slow down, or change direction). f. the greater the mass of an object the more force is needed to achieve the same change in motion. g. the role of gravity in forming and maintaining planets, stars and the solar system.